Aggrecan is the major proteoglycan of cartilage and provides this tissue with its mechanical properties of compressibility and elasticity. In arthritic conditions one of the earliest changes observed in cartilage morphology is the depletion of aggrecan [Mankin et al. (1970) J. Bone Joint Surg. 52A, 424–434], which appears to be due to an increased rate of degradation.
The aggrecan molecule is composed of two N-terminal globular domains, G1 and G2, which are separated by an approximately 150 residue interglobular domain (IGD), followed by a long central glycosaminoglycan (GAG) attachment region and a C-terminal globular domain, G3 [Hardingham et al. (1992) in Articular Cartilage and Osteoarthritis: Aggrecan, The Chondroitin Sulfate/Keratin Sulfate Proteoglycan from Cartilage (Kuettner et al.) pp. 5–20, Raven Press, New York and Paulson et al. (1987) Biochem. J. 245, 763–772]. These aggrecan molecules interact through the G1 domain with hyaluronic acid and a link protein to form large molecular weight aggregates which are trapped within the cartilage matrix [Hardingham et al. (1972) Biochim. Biophys. Acta 279, 401–405, Heinegard et al. (1974) J. Biol. Chem. 249, 4250–4256, and Hardingham, T. E. (1979) Biochem. J. 177, 237–247]. Loss of aggrecan from cartilage in arthritic conditions involves proteolytic cleavage of the aggrecan core protein within the IGD, producing a N-terminal G-1 fragment that remains bound to hyaluronic acid and the link protein within the matrix, releasing a large C-terminal GAG-containing aggrecan fragment that diffuses out of the cartilage matrix. Loss of the C-terminal fragment results in cartilage deficient in its mechanical properties. This deficiency arises because the GAGs are the components of aggrecan that impart the mechanical properties to the molecule through their high negative charge and water binding capacity.
Two major sites of proteolytic cleavage have been identified within the IGD, one between amino acid residues Asn341-Phe342 and the other between amino acid residues Glu373-Ala374. Although G1 fragments formed by cleavage at the Asn341-Phe342 site and at the Glu373-Ala374 site have been identified within articular cartilage [Flannery et al. (1992) J. Biol. Chem. 267, 1008–1014], the N-terminus identified on the large GAG-containing aggrecan C-terminal fragments in synovial fluids of patients with osteoarthritis [Sandy et al. (1992) J. Clin. Invest. 69, 1512–1516], inflammatory joint disease [Lohmander et al. (1993) Arthritis Rheum. 36, 1214–1222] and in the media from cartilage explant and chondrocyte cultures stimulated with interleukin-1 or retinoic acid [Sandy et al. (1991) J. Biol. Chem. 266, 8198., Sandy et al. (1991) J. Biol. Chem. 266, 8683–8685., Loulakis et al. (1992) Biochem. J. 264, 589–593., Ilic et al. (1992) Arch. Biochem. Biophys. 294, 115–122., Lark et al. (1995) J. Biol. Chem. 270, 2550–2556.] was ARGSVIL, indicating that they were formed by cleavage between amino acid residues Glu373-Ala374. These observations suggest that cleavage at this site may be responsible for cartilage degradation.
Although many matrix metalloproteases (MMP-1, -2, -3, -7, -8, -9 and 13) have been shown to cleave in vitro at the Asn341-Phe342 site, digestion of aggrecan with a number of these purified proteases has not resulted in cleavage at the Glu373-Ala374 site [Fosang et al. (1992) J. Biol. Chem. 267, 19470–19474., Flannery et al. (1992) J. Biol. Chem. 267, 1008–1014., Fosang et al. (1993) Biochem. J. 295, 273–276., Fosang et al. (1996) FEBS Lett. 380, 17–20., Flannery et al. (1993) Orthop. Trans. 17, 677., and Fosang et al. (1994) Biochem. J. 305, 347–351]. Therefore, cleavage at this site has been attributed to a novel, proteolytic activity, “aggrecanase”.
In addition to the Glu373-Ala374 bond within the interglobular domain of aggrecan, four potential aggrecanase-sensitive sites have been identified within the C-terminus of the aggrecan core protein [Loulakis et al. (1992) Biochem. J. 264, 589–593. and Sandy et al. (1995) Acta Orhtop Scand (Suppl 266) 66, 26–32]. Although cleavage at these sites which are not within the interglobular domain would not be expected to release the major portion of the aggrecan molecule from the matrix, they may be involved in earlier processing of aggrecan within the matrix.